When a task calls for consistent, large amounts of power output, hydraulic actuation is a popular choice. However, for certain systems that require short bursts of high power, followed by a period of low power, the inefficiencies of hydraulics become apparent. One system that fits this description is a legged robot. McKibben muscles prove to be a wise choice for use on legged robots due to their light weight, high force capability, and inherent compliance. Variable recruitment, another novel concept for hydraulic actuation, offers the ability to further improve efficiency for hydraulic systems. This paper will discuss the efficiency characterization of variable recruitment McKibben muscles intended for use on a bipedal robot, but will focus on the novel test apparatus to do so. This device is a hydraulic linear dynamometer that will be controlled such that the muscles experience similar force-stroke levels to what will be required on a bipedal robot. The position of the dynamometer’s drive cylinder will be controlled so that the muscles experience the proper position trajectory that will be needed on the robot. The pressure of the muscles will be controlled such that the force they experience will mimic the forces that occur on the robot while walking. Hence, these dynamic tests will ensure that the muscle bundles will meet the force-stroke requirements for the given robot. Once these muscle bundles are integrated onto the walking robot, the power savings of variable recruitment McKibben muscle bundles compared to the traditional hydraulic system will be demonstrated.

McKibben artificial muscles are often utilized in mobile robotic applications that require compliant and light weight actuation capable of producing large forces. In order to increase the endurance of these mobile robotic platforms, actuation efficiency must be addressed. Since pneumatic systems are rarely more than 30% efficient due to the compressibility of the working fluid, the McKibben muscles are hydraulically powered. Additionally, these McKibben artificial muscles utilize an inelastic bladder to reduce the energy losses associated with elastic energy storage in the usual rubber tube bladders. The largest energy losses in traditional valve-controlled hydraulic systems are found in the valving implementation to match the required loads. This is performed by throttling, which results in large pressure drops over the control valves and significant fluid power being wasted as heat. This paper discusses how these throttling losses are reduced by grouping multiple artificial muscles to form a muscle bundle where, like in skeletal muscle, more elements that make up the muscle bundle are recruited to match the load. This greatly lessens the pressure drops by effectively changing the actuator area, leading to much higher efficiencies over a broader operation envelope. Simulations of several different loading scenarios are discussed that reveal the benefits of such an actuation scheme.

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